In considering the human visual system, most people immediately think about the eyes and the constituent parts, such as the cornea, pupil, eye lens, the vitreous humor, and the retina with the rods and cones. However, the human visual system also includes neural processing that enables image interpretation and understanding. Such neural processing generally occurs automatically, without any conscious consideration by the observer. For example, the images formed on the retina of each eye are upside down, but the visual system automatically provides orientational corrections. As another example, the ratios of the number of short or blue cones, the number of medium or green cones, and the number of long or red cones varies widely among individuals, as does the organization of these cones within the retina, whether randomly dispersed or clustered. The green cones are also much more light sensitive than the red or blue cones. Yet, by and large, most people perceive the various color shades in a sufficiently comparable way that we can agree upon the colors present in a scene.
The structure and response attributes of the human visual system impact perception; for example, in cinema, the temporal processing limits allow people to perceive continuous motion from a series of still frame images presented at 30 frames/sec (fps). By comparison, human perception of optical illusions exploit gaps or expectations in our automatic cognitive visual system, such that we can perceive visual content or sensations within the image content which is often not actually present in the content itself. An optical illusion is characterized by the visual perception of image content that differ from objective reality. The information gathered by the eye is processed in the brain to give a perception that does not completely correlate with a physical measurement of the stimulus source or image. Certainly, the perception of optical illusions varies on an individual basis, depending on visual sensitivity and impairments (such as color blindness). The perception of optical illusion can also depend on age and cultural influences. For example, children tend not to perceive the well-known Ebbinghaus illusion as strongly as adults do, while non-Western and rural people, who are less accustomed to interpreting depth and flatness cues from two-dimensional (2D) images, can be less susceptible to the well-known Ponzo illusion. While human observers often respond to optical illusions attentively and with puzzlement or bemusement, some illusions can induce discomfort or nausea in certain observers.
Optical illusions can be characterized to include cognitive optical illusions, in which the eye and brain make unconscious inferences, and physiological illusions, in which the eyes and brain are affected by excessive stimulation of a specific type (e.g., brightness, tilt, color or motion). Examples of cognitive illusions include illusions of perspective, such as the Penrose Stairs or the drawings by M. C. Escher. These images exploit our expectations of how a three-dimensional view is illustrated in two-dimensions, by cleverly misapplying cues of perspective and shading to depict objects that are physically impossible. The Ponzo illusion similarly exploits our expectations that convergent lines are associated with distance; to have two lines intersecting the convergent lines appear of different length, even though their lengths are identical.
While many physiological optical illusions have been created, their perceptive impact on the human visual system has not been fully explained. The article “Uncertainty in visual processes predicts geometrical optical illusions”, by Cornelia Fermüller et al. (Vision Research, Vol. 44, pp. 727-749, 2004) proposes that uncertainty or noise in the human visual system that relates to determining intensity, positions, and orientations of image features introduces systemic errors that cause perceptual anomalies compared to the original image. For example, interpretive biases relative to perception of edge elements and intersection points may explain many geometrical illusions, while biases in interpreting motion or optical flow may cause motion illusions such as the Ouchi illusion (see FIG. 6A). Generalizing, it is noted that there are many physiological optical illusions, including the well-known Hermann Grid and scintillating grid illusions, or the well-known pulsing vortex, rotating snakes, and flowing leaves illusions, that can be classified as apparent motion optical illusions which provide temporally variant sensations, such as pulsation and movement, including rotation, expansion, and contraction, that are not actually present in the images themselves, which are static in nature. Such illusions can be observed whether they are printed or presented statically on an electronic display. The temporal effects of these illusions can generally be attributed to unconscious interpretive bias in image interpretation that intersects with temporal phenomena of the human visual system such as peripheral drift, neural lags, fixation and after image effects, and saccadic eye motions.
There are also physiological optical illusions that specifically exploit attributes of human color perception, such as color and contrast adjacency effects, to fool the eye in seeing colors and grayscale patches incorrectly. The article, “A Brief Classification of Colour Illusions’, by A. Kitaoka (Colour: Design & Creativity, Vol. 5, pp. 1-9, 2010), gives many examples of color perception optical illusions. For example, there are illusions of color or contrast differences that exploit adjacency interactions that are embedded in human lateral signal processing. These illusions reveal perceptual difference in color or contrast perception of an area when adjacent areas have different luminosities or color content. For example, with the Munker-White illusion, the perception of mid-tone neutral (grey) patches is altered by the presence of strong luminance shifts (darker or lighter) in neighboring image content. The Munker illusion illustrates a similar effect on color perception, where the perception of identical color patches is altered by the presence of different adjacent colors near one color patch as compared to another.
While some optical illusions, such as the Ouchi illusion or the Fraser-Wilcox illusions, are motion illusions that have a perceptual impact as black and white or grayscale images, their effects can be enhanced when color is also used. For example, A. Kitaoka has published integrated illusions, such as peripheral drift illusions and the “rotating snakes” illusion, which combines color selection with other illusions, such as the optimized Fraser-Wilcox and the Rotating Ouchi illusion to create illusions that can be more visually compelling than the black and white parent illusions. The paper “The effect of color on the optimized Fraser-Wilcox illusion”, by A. Kitaoka (9th L'OREAL Art and Science of Color Prize, pp. 1-16, 2006) discusses how color and contrast manipulation can change the visual impact of the optimized Fraser-Wilcox illusion, including changing the direction and rate or magnitude of the apparent rotation of the image. The paper “Illusory motion from change over time in the response to contrast and luminance”, by B. Backus et al. (Journal of Vision, Vol. 5, pp. 1055-1069, 2005) proposes that optical illusions of motion are generated when viewing static repeated asymmetric patterns due to fast and slow changes over time in the neuronal representation of contrast or luminance. The impact of other temporal illusions, such as the pulsing vortex illusion, or the Hermann grid and scintillating grid illusions, the latter of which cause the perception of pulsing at the intersections of a white (or light-colored) grid on a black background, can also be altered by changing the pattern of colors and contrast within the illusionary image.
At present, optical illusions are generally available as collections in books or on websites, and their design and physiological basis is discussed in academic papers. In general, optical illusions are primarily used to amuse or entertain viewers, including in art and architecture. With their unusual visual trickery, optical illusions often hold an observer's attention for prolonged time periods, while the observer is both amused and puzzled by the visual effects. Optical illusions are also used as tools in vision research to test or reveal the cognitive or perceptual mechanisms, interactions, or clinical deficiencies of the human visual system.
Otherwise, the human perception of optical illusions has been used for little practical effect, although their selective use as a “reverse Turing test”, has been proposed in the paper “Practical Application of Visual Illusions: errare humanum est” by G. Brelstaff et al (Proc. 2nd Symposium on Applied Perception in Graphics and Visualization, p. 161, 2005). A Turing test is a test of a machine's ability to exhibit intelligent behavior, and a machine passes the test if its function, at least in a limited way, is indistinguishable from that of a human. By comparison, a reverse Turing test is a test to distinguish humans from computers and other forms of artificial or alien intelligence. The one widely used version of a reverse Turing test practiced today, involves the use of a “CAPTCHA”, a “Completely Automated Public Turing test to tell Computers and Humans Apart”, as a visual word recognition challenge-response test. CAPTCHAs are widely used in computing to attempt to ensure that a response is generated by a human rather than a computer. For example, as humans can readily interpret the confusing image content of a CAPTCHA, while computer algorithms have difficulty, humans can gain website access while automated programs are restrained. The Belstraff paper proposes that as humans innately can perceive optical illusions, such as Kitaoka's Rotating Snakes, and machine vision systems cannot, that the human perceptual reaction to static optical illusion images can be used to discriminate the presence of human intelligence. As a variant, it has also been proposed to use text illusions as CAPTCHAs, as human readable steganography, in which smaller case static text is hidden within larger case static text.
Thus, the opportunity exists to present, create, and alter optical illusion images for various practical and previously unforeseen effects. In particular, printed optical illusion images which are dynamic by the use of mutable inks can provide novel functional value.